I have a PIC32 microcontroller (MCU) system that experienced intermittent (though regularly) resets, most likely due to ground bounce caused by the switching of inductive loads (solenoids). All the decoupling capacitors and bulk capacitor are present as per the datasheet's recommendation. The MCU is contained on a breakout board that plugs in to the rest of the circuitry. For modularity and ease of replacement if necessary.
One solution that nearly eliminated the resets was to put a very large value capacitor across the main MCU power pins of the breakout board. The MCU is powered by 3.3 V. In this particular case, it was a 1000 uF electrolytic capacitor with a 25 V rating (which is a rather large capacitor). However, the odd intermittent resets still occurs now and then, but not nearly as much as it did without the electrolytic capacitor. I did some testing with a power supply and LED where I removed the power to the breakout board, and the LED still remained powered on for a couple of seconds, as expected. However, it seems that there is a saturation point at which more capacitance doesn't help the situation anymore.
Herewith follows a few questions:
- Is there a saturation point at which any larger capacitors will not help the situation (i.e. that remains charged for longer)?
- Does the fact that the capacitor is rated for 25 V make it a bad choice for such a bulk capacitor for use at only 3.3 V?
- Is there a minimum voltage at which a particular capacitor will reliably provide adequate stability as a temporary power supply in the event of a ground bounce occurrence?
- Would a super capacitor be a better choice in this case (particularly the ones with low rated voltage and high capacitance in the Farad-range)?
- Taking all the above questions into consideration, what is the best way to provide an MCU with a reliable, stable power supply to eliminate susceptibility to ground bounce (other than using external power supplies like switch mode supplies).
Best Answer
You want to attempt to create a star ground where the power supply ground connection is in between the power and digital sides of the circuit. Ground bounce is caused by resistive losses when a large current flows through the ground traces. By putting the power circuitry on the "other" side of a star ground, you put the resistor that is causing the drop/bounce in series with the power load and not in series with the digital load.
Given your construction this might or might not be practical.